1. Field of the Invention
The present invention relates to oxygen absorptive resins and resin compositions used for packaging materials, containers, and the like for products that are highly susceptible to degradation by oxygen, such as foods, drinks, medicines, cosmetics, and the like. The present invention also relates to oxygen absorptive resin compositions having good gas barrier properties, good moisture-resistance, good aroma retentivity, and good flavor barrier properties, in addition to good oxygen absorbency. The present invention further relates to molded articles using such resins or compositions, such as packaging materials, containers, and the like for foods, drinks, medicines, cosmetics, and the like.
2. Description of the Related Art
As well known, it is possible to maintain the quality of products susceptible to degradation by oxygen and prolong the shelf life thereof by protecting the products from exposure to oxygen. For example, a food liable to degrade in an oxygen atmosphere is maintained in quality and retarded from spoiling by packaging the food to block it from exposure to oxygen. Such packaging is also advantageous in prolonging the value of the product and thus reducing the cost that may be required for disposal of a degraded product and the repurchase of a new product. Several techniques for minimizing exposure to oxygen have been developed in the food packaging industry. Examples of such means generally available at present are modified atmosphere packaging (MAP), vacuum packaging, and oxygen barrier film packaging. The former two means utilize an atmosphere with a low concentration of oxygen, while the last means physically blocks oxygen from entering the packaged environment.
The oxygen barrier film packaging, which is classically most used, utilizes various types of gas barrier resins as a film material.
Extensively used gas barrier resins extensively are an ethylene-vinyl alcohol copolymer (hereinafter may be referred to as EVOH), polyamide, polyvinyl chloride, polyacrylonitrile, and the like. These resins have good oxygen or carbon dioxide gas barrier properties and can be melt-molded. They therefore have a wide range of applications such as packaging films, sheets, bottles, and containers. These resins are laminated with thermoplastic resins, in particular, polyolefin resins, having good moisture-resistance, mechanical properties, and the like, to form multilayered plastic packaging materials. Such multilayered plastic packaging materials are broadly used as containers that have excellent oxygen barrier properties in the form of bags, bottles, cups, and pouches, for example, in a variety of fields such as foods, cosmetics, medicines, chemicals, and toiletries.
The above packaging materials using the gas barrier resins have good barrier properties against oxygen and carbon dioxide gas. However, the permeability thereof against gas such as oxygen gas is not exactly zero or near zero, unlike metals used for cans and the like and glass used for bottles and the like, but still permit transmission of an amount of gas that is not negligible. As for packaging materials for foods, in particular, of great concern is degradation of foods due to oxidation when they are preserved for an extended period of time. Therefore, improvement on the oxygen barrier properties of the resin materials is strongly desired. Also desired is scavenging of oxygen existing in a packaging container that has entered the container together with the contents during packaging and filling of the contents, when the contents are susceptible to oxidation, so as to protect them from being degraded. For this purpose, it is proposed to enclose an oxygen scavenger in the inner environment of the package, or to give the oxygen scavenging function to the gas barrier resin by mixing an oxygen scavenger in the gas barrier resin. The latter proposal of giving the oxygen scavenging function to the gas barrier resin, in particular, has an effect of scavenging oxygen uniformly over the entire inner environment of the package. Additionally, it also has an effect of scavenging oxygen that is being transmitted through the wall of the package by the scavenger mixed in the resin material. This makes it possible to minimize the level of oxygen content within the entire inner environment of the package.
Various compounds are known to be oxygen scavengers. As an improved oxygen scavenger, a composition including a transition metal catalyst and an ethylenically unsatarated compound (e.g., polybutadiene and polyisoprene) is proposed, for example, in Japanese Laid-Open Patent Publication No. 5-115776. This literature exemplifies trans-1,4-polyisoprene and 1,2-polybutadiene as the ethylenically unsatarated compound, and a cobalt compound as the transition metal catalyst. However, sufficient oxygen scavenging effect of the disclosed oxygen scavenger is not so sufficiently obtained. Moreover, in some cases, the delay until the start of effective oxygen scavenging (the induction period) is disadvantageously long.
As the method of giving the oxygen absorption function (i.e., scavenging function) to a gas barrier resin, the following methods are proposed: (1) An oxidation catalyst such as a transition metal is added to EVOH to facilitate oxidation of the EVOH so that oxygen can react with the EVOH as it is transmitted in a film composed of the EVOH, thereby giving the oxygen absorption function to the EVOH (Japanese Laid-Open Patent Publication No. 4-211444); (2) An oxidation catalyst such as a transition metal is added to polyvinyl chloride to facilitate oxidation of the polyvinyl chloride so that oxygen can react with the polyvinyl chloride as it is transmitted in a film composed of the polyvinyl chloride, thereby giving the oxygen absorption function to the polyvinyl chloride (Japanese Laid-Open Patent Publication No. 4-45144); (3) A mixture substantially composed of polyolefin and an oxidation catalyst is dispersed in EVOH so that oxygen can react with the polyolefin in the EVOH as it is transmitted in a film of EVOH containing the mixture, thereby obtaining a resin composition having the oxygen absorption function (Japanese Laid-Open Patent Publication No. 05-156095); and (4) EVOH, polyolefin, and an oxidation catalyst are blended so that oxygen can react with the polyolefin and the EVOH as it is transmitted in a film of the mixture of the EVOH and polyolefin, thereby obtaining a resin composition having the oxygen absorption function (Japanese Laid-Open Patent Publication No. 05-170980).
However, methods (1) and (2) fail to provide sufficiently improved oxygen barrier properties, though. Moreover, the transparency is not adequate due to the addition of the large amount of oxidation catalyst. Methods (3) and (4) also have a drawback in that the transparency is reduced due to the addition of polyolefin to the gas barrier resin.